FA-7221 / Transport flow control / Open access
Fin consumes one sequence number · case 01
Fin consumes one sequence number.
ROOT CAUSE
The faulty implementation uses `payload` for the fin-consumes-one-sequence-number decision.
VERIFIED REPAIR
Apply the explicit fin-consumes-one-sequence-number contract, including the tested boundary and negative cases.
Unsuccessful approach: The attempted repair uses `payload+1` and still violates a separate fixture.
Case contract
Bounded offline decision model: fin consumes one sequence number. Inputs are already validated protocol fields; the explicit fixtures define the supported policy, not a complete protocol implementation.
Why this case matters
This deterministic transport flow control model isolates the fin-consumes-one-sequence-number policy at a network component boundary.
1 / The failure
Exit 1"""Failure Map reference implementation. Python standard library only."""
import json
import ipaddress
import re
from urllib.parse import urlsplit, urljoin
N = 1
observations = []
def solve(payload, fin):
return payload
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve(3, True), 4)", "check('fixture 2', solve(3, False), 3)", "check('fixture 3', solve(0, True), 1)", "check('fixture 4', solve(0, False), 0)"]
for _line in _fixtures[N % len(_fixtures):] + _fixtures[:N % len(_fixtures)]:
exec(_line)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
| Boundary fixture | Actual | Expected | Outcome |
|---|---|---|---|
| fixture 2 | 3 | 3 | Passed |
| fixture 3 | 0 | 1 | Failed |
| fixture 4 | 0 | 0 | Passed |
| fixture 1 | 3 | 4 | Failed |
SHA-256 / 8c2497213bb33514b298fde01ae9da649cbc1cdf0ae0ef9f49e98248054d780f
2 / The unsuccessful fix
Exit 1"""Failure Map reference implementation. Python standard library only."""
import json
import ipaddress
import re
from urllib.parse import urlsplit, urljoin
N = 1
observations = []
def solve(payload, fin):
return payload+1
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve(3, True), 4)", "check('fixture 2', solve(3, False), 3)", "check('fixture 3', solve(0, True), 1)", "check('fixture 4', solve(0, False), 0)"]
for _line in _fixtures[N % len(_fixtures):] + _fixtures[:N % len(_fixtures)]:
exec(_line)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
| Boundary fixture | Actual | Expected | Outcome |
|---|---|---|---|
| fixture 2 | 4 | 3 | Failed |
| fixture 3 | 1 | 1 | Passed |
| fixture 4 | 1 | 0 | Failed |
| fixture 1 | 4 | 4 | Passed |
SHA-256 / 6a1c26f86acfaaf718318a8387d1484072379f511a326fd2dc0a1d753c5b5cca
3 / The verified repair
Exit 0"""Failure Map reference implementation. Python standard library only."""
import json
import ipaddress
import re
from urllib.parse import urlsplit, urljoin
N = 1
observations = []
def solve(payload, fin):
return payload + int(fin)
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve(3, True), 4)", "check('fixture 2', solve(3, False), 3)", "check('fixture 3', solve(0, True), 1)", "check('fixture 4', solve(0, False), 0)"]
for _line in _fixtures[N % len(_fixtures):] + _fixtures[:N % len(_fixtures)]:
exec(_line)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
| Boundary fixture | Actual | Expected | Outcome |
|---|---|---|---|
| fixture 2 | 3 | 3 | Passed |
| fixture 3 | 1 | 1 | Passed |
| fixture 4 | 0 | 0 | Passed |
| fixture 1 | 4 | 4 | Passed |
SHA-256 / c771483a17859d285e301944f3b820f64f352c79da736ebf11587d2f53ce8c48
Verification & scope
Bounded deterministic policy model over validated inputs; not a complete protocol stack or an interoperability claim. The five variants rotate the same explicit fixture set. This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.
Observations recorded using Python 3.12.14 at 2026-09-29T14:38:09.526069+00:00.
Case digest / 112a407d65896bb2602234702edcd70ed06602fd8a70c4f6945220697d3e196c